Pulse generator for an ultrasound flowmeter

ABSTRACT

A pulse generator including a control logic operating, by control pulses, a switch connected on the input side with a voltage supply unit and delivering a pulse voltage on the output side, there is connected, between control logic and the switch, a capacitor, which decreases the pulse voltage when the pulse frequencies are too low, and, additionally, arranged between voltage supply unit and switch, an RC-member, which decreases the pulse voltages when the repetition rates of the control pulses are too high, in order to enable use of the pulse generator in the explosion-protected area.

FIELD OF THE INVENTION

The invention relates to a pulse generator comprising a control logic,which drives by means of control pulses a switch, which is connected onthe input side with a voltage supply unit and delivers a pulse voltageon the output side.

BACKGROUND OF THE INVENTION

Such pulse generators are used in many applications in the field ofprocess automation with reference to its applications of ultrasonics andradar.

Thus, the ultrasonic flow measuring devices permit, in a simple manner,contact-less determination of volume flow rate in a pipeline.

The known ultrasonic flow measuring devices work either on the basis ofthe Doppler principle or the travel time difference principle.

In the case of the travel time difference principle, the differenttravel times of ultrasonic pulses are evaluated relative to the flowdirection of the liquid.

For this purpose, ultrasonic pulses are transmitted both in thedirection of flow and opposite thereto. From the travel time difference,the flow velocity can be determined, and, with known diameter of thepipeline cross section, the volume flow rate.

In the case of the Doppler principle, ultrasonic waves of knownfrequency are coupled into the liquid, and the ultrasonic wavesreflected by the liquid are evaluated. From the frequency shift betweenthe waves coupled in, and those reflected, one can likewise determinethe flow velocity of the liquid.

Reflections in the liquid occur, however, only when air bubbles orimpurities are present therein, so that this principle finds use mainlyin the case of contaminated liquids.

The ultrasonic waves are produced, or received, as the case may be, withthe help of so-called ultrasonic transducers. For this purpose,ultrasonic transducers are placed securely on the pipe wall of thepipeline section of concern. More recently, clamp-on ultrasonicmeasuring systems are also obtainable. In the case of these systems, theultrasonic transducers are held against the tube wall using only a clampfastener. Such systems are known e.g. from EP-B-686 255, and from U.S.Pat. Nos. 4,484,478 and 4,598,593.

Another ultrasonic flow measuring device, which works on the basis ofthe travel time difference, is known from U.S. Pat. No. 5,052,230. Thetravel time in this case is determined using bursts, i.e. shortultrasonic pulses.

The ultrasonic transducers are usually made of a piezoelement and acoupling wedge. Ultrasonic waves are produced in the piezoelement andguided by way of the coupling wedge to the pipe wall, and, from there,into the liquid. Since the sound velocities in liquids and plastics aredifferent, the ultrasonic waves are refracted at the transition from onemedium to the other. The refraction angle is determined by Snell's law.The refraction angle is, consequently, dependent on the ratio of thepropagation velocities in the two media.

Often, ultrasonic flow measuring devices are used in explosion-protectedareas. In these areas, ignitable gases are present, whose ignition is tobe avoided. For explosion-protected areas, there are correspondingsafety specifications, in order to prevent endangerment of plant andpersons. An ignition of the gases can happen, when certain values ofoutwardly-acting voltage, current, inductance or capacitance areexceeded and, consequently, sufficient energy is introduced into thegas, that an ignition process is triggered. High pulse-voltages areneeded to get a sufficient measurement accuracy. With smallpulse-voltages, signal evaluation becomes extremely difficult. Ex-areasare divided in the known safety standard into different zones, which aregoverned by different safety criteria. In the case of malfunctions ofthe control logic that governs the production of the voltage pulses, itis not out of the question that the number of the pulse periods perburst will become too large, the pulse frequency too high, or the burstrepetition rate too high. All of these cases can lead to a gas ignition.The situation, wherein the voltage of the voltage supply unit is alwaysat the output, must likewise be prevented.

An object of the present invention is, therefore, to provide a pulsegenerator suited especially for ultrasonic flow measuring devices andalso permitting a safe application in Ex-areas.

This object is achieved by a pulse generator with at least one capacitorbetween the control logic and the switch, which decreases the pulsevoltage when the pulse frequency of the control pulses is too small, andbetween the voltage control unit and the switch, an RC-member isarranged, which decreases the pulse voltages, when the repetition rateof the control pulses is to high.

SUMMARY OF THE INVENTION

An essential idea of the invention is to arrange the RC-member betweenthe voltage supply unit and the switch, in order to decrease the pulsevoltage, when the switch is driven at a too high repetition rate, and toarrange a capacitor between the control logic and the switch, in orderto decrease the pulse voltage in the case where the pulse frequencies ofthe control pulses are too low.

The invention will now be explained in greater detail on the basis of anexample of an embodiment presented in the drawings, whose figures showas follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an ultrasonic flow measuring device;

FIG. 2 is a schematic circuit diagram of a pulse generator of theinvention for ultrasonic flow measuring devices; and

FIGS. 3 a, 3 b, 3 c, are pulse-voltage versus time diagrams fordifferent behaviors of the control logic.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, in greatly simplified manner, an ultrasonic flow measuringdevice possessing two ultrasonic transducers 2, 3, which are arrangedaxially-parallel and displaced from one another on the outer wall of apipeline 1. The liquid F in the pipeline 1 is flowing in the directionof the arrow.

This transducer pair can be operated in two different ways. Either theultrasonic transducer 2 acts as transmitting transducer and theultrasonic transducer 3 as the receiving transducer, or the ultrasonictransducer 2 as the receiving transducer and the ultrasonic transducer 3as the transmitting transducer, so that, alternatingly, measurement isin the flow direction or opposite to the flow direction.

Each ultrasonic transducer 2, 3 is composed of a piezoelement P2, P3 anda coupling element 21, 31, which either couples the ultrasonic signalsat an angle á less than 90-degrees into or out of the wall of thepipeline. The angle á is chosen such that as flat an angle as possibleis obtained in the medium while simultaneously being able to couple theultrasound into as many pipe materials as possible without totalreflection.

The piezoelements P2, P3 transduce either ultrasonic pulses intomechanical oscillations, which are the actual ultrasonic signals, or thereverse, mechanical oscillations into electrical oscillations.

Both ultrasonic transducers 2, 3 are connected over leads 23, 33 with ameasuring circuit 100, which includes a pulse generator. The electricalpulses are fed over the leads 23, 33.

The voltage pulses, with which the piezoelements are driven, areproduced with the help of a pulse generator P, which essentiallycomprises a control logic SL, a switch SR and a voltage supply unit S(FIG. 2). The control logic SL delivers the control signals whichoperate the switch SR. The voltage pulses are produced by closing andopening of the switch SR. The amplitude of the pulse voltage isdetermined in normal operating condition by the output voltage of thevoltage supply unit S.

The control logic permits setting of the number of periods per burst,and the pulse frequency and the repetition rate of the bursts. Thesesettings are variable and stored in a data record in the control logicSL. The settings can be changed with the help of a microprocessor μC.

The voltage supply unit S is connected, by way of an RC-member composedof a resistance R1 and a capacitor C1, with an input E1 of a fieldeffect transistor (FET), which serves as the switch SR. The controllogic SL is also connected with the field effect transistor (FET), atits second input E2, via two capacitors C01 and C02. The output A1 ofthe field effect transistor FET is connected over a resistance A3, withthe ultrasonic transducer 2. The ultrasonic transducer 2 is composedessentially of a piezoelement, which is in circuit with additional,passive or active components (resistances, inductances, diodes). Thecontrol logic SL is driven by the microprocessor μC. Since the pulsegenerator P produces bipolar voltage pulses, the upper part of thecircuit is mirrored below.

The operation of the invention will now be explained in more detail.

The control logic SL delivers the control pulses which operate theswitch SR. The control logic SL is normally adjusted such that thevoltage pulses are produced as bursts of repetition rate 1–10milliseconds and pulse frequency of some hundreds of kilohertz. FIG. 3 ashows a corresponding pulse-voltage versus time diagram. The pulsefrequency is here 500 kilohertz. A burst has 5 pulses, and therepetition rate of the bursts is 5 milliseconds, The amplitude of thepulse voltage lies at 30 volts, which is the output voltage of thevoltage supply unit S.

A possible malfunction of the control logic SL can lead to the pulsefrequency being too small. Such a case is shown in FIG. 3 b, where thepulse frequency amounts to only 50 kilohertz. Clearly evident is thatthe amplitude of the pulse voltage declines sharply with each individualpulse. Already after 5 pulses, the pulse voltage has decreased by morethan half of the original starting value.

Another malfunction of the control logic SL can lead to the repetitionrate being too high. Such a case is illustrated in FIG. 3 c. Here, therepetition rate amounts to 100 microseconds. The amplitude of the pulsevoltage lies now only at about 5 volts.

With the help of the pulse generator of the invention, despite the caseof a malfunction of the control logic SL, a safe operation is possibleeven in explosion-protected areas.

The pulse generator of the invention can be applied in a multiplicity ofways, not only in the ultrasonic region, but also in the radar region,etc.

1. A pulse generator comprising: a switch; a voltage supply unitconnected to the input side of said switch; a control logic, whichdrives by means of control pulses said switch, said switch delivers apulse voltage on its output side; at least one capacitor located betweensaid control logic and said switch, which decreases the pulse voltagewhen the pulse frequency of the control pulses is too small; and an R-Cmember located between said voltage supply unit and said switch, saidRC-member said decreases the pulse voltages, when the repetition rate ofthe control pulses is too high.
 2. For ultrasonic flow measuringdevices, a pulse generator as claimed in claim 1, wherein: said switchis a field effect transistor.
 3. The pulse generator as claimed in claim1, wherein: said RC-member has a time-constant making an ignitionimpossible.
 4. The pulse generator as claimed in claim 1, wherein: twocapacitors are provided which are connected in series.
 5. The pulsegenerator as claimed in claim 1, further comprising: a current-limitingoutput resistance connected after said switch.
 6. A pulse generator asclaimed in claim 1, wherein: bipolar voltage pulses are produced.
 7. Apulse generator as claimed in claim 1, wherein: it is applied forultrasonic flow measuring devices.